A typical one of surface light source devices used for portable telephones, portable electronic terminal units and so forth, has a fluorescent lamp on a side of the side face (on the side of an incidence surface) of a light guide plate as a light source, to emit light to a plane from the fluorescent lamp through the light guide plate to irradiate a liquid crystal display panel with the exiting light. Such a surface light source device is combined with the liquid crystal display panel to form an image display unit. In order to improve the luminance of the surface light source device, various devices have been made to the light guide plate. As one of them, the applicant of the present application has proposed a surface light source device wherein a large number of prisms-like protrusions extending in directions substantially perpendicular to an incidence surface of a light guide plate (see, e.g., Japanese Patent Laid-Open No. 10-268138).
By the way, in recent years, in order to improve the user's handling of portable telephones, portable electronic terminal units and so forth, it has been attempted to increase a display screen as large as possible while decreasing the thickness and weight of the telephone or unit body. In such a technical background, there has been developed a surface light source device which uses light emitting diodes (LEDs) as point light sources to decrease a light source housing space to increase the size of a display screen by the decrease of the light source housing space. In addition, portable telephones, portable electronic terminal units and so forth using the surface light source device have been put on the market.
FIGS. 40 and 41 show an example of a surface light source device 101 using LEDs 100 as light sources in the above described light guide plate having a large number of prism-like protrusions. As shown in these figures, the surface light source device 101 has a plurality of LEDs 100 which face a side face (an incidence surface) 103 of a light guide plate 102. Aback surface (a surface opposite to an emission surface 106) of the surface light source device 101 is formed with a large number of prism protrusions 104 extending in directions substantially perpendicular to the incidence surface 103 of the light guide plate 102. The prism protrusions 104 are designed to reflect light L, which is propagated through the light guide plate 102, to condense the light L, which leaves the emission surface 106 of the light guide plate 102, in a direction substantially normal to the emission surface 106 to enhance luminance (see FIG. 42).
However, if a fluorescent lamp is substituted for the LEDs 100 as a light source for such a light guide plate 102, there are some cases where V-shaped abnormal emissions (bright lines) H are produced at positions, which substantially correspond to the LEDs 100, in the vicinity of the incidence surface 103 to remarkably damage the quality of illumination when the light guide plate 102 is viewed from the side of the emission surface 106 as shown in FIG. 40. The reasons for this are considered that the LEDs 100 have directivity unlike fluorescent lamps and that the light L propagated through the light guide plate 102 is easily reflected on the prism protrusions 104, which are positioned in the vicinity of the incidence surface, to leave the emission surface in a specific direction.
As shown in FIGS. 43 and 44, in a surface light source device for use in a portable telephone, portable electronic terminal unit or the like, a fluorescent lamp 100A is arranged on the side of the side face (the incidence surface 103) of the light guide plate 102 for emitting light to a plane through the light guide plate 102 to irradiate a liquid display panel (not shown) with the emitted light. Such a surface light source device 101 is combined with the liquid crystal display panel (not shown), which is arranged so as to face the emission surface 106 of the light guide plate 102, to form an image display unit. In such a surface light source device 101, the light guide plate has been conventionally devised in various ways. For example, a large number of prism protrusions 104 extending in directions substantially perpendicular to the incidence surface 103 of the light guide plate 102 are formed on the back surface 105 of the light guide plate 102 to utilize the light reflecting function of the prism protrusions 104 for enhancing the luminance of illuminating light (see, e.g., Japanese Patent Laid-Open No. 10-268138).
However, in such a surface light source device 101 using the fluorescent lamp 100A as a linear light source, there is a structural problem in that darker portions (low luminance portions 110 shown by slant lines in FIG. 43) than other portions are easily caused in both end portions on the side of the incidence surface 103 of the light guide plate 102 since electrode portions positioned in both end portions of the fluorescent lamp 100A do not emit light. As shown in FIGS. 43 and 44, in such a surface light source device, there is a problem in that light beams H1 and H2 incident on upper and lower edge portions 103A and 103B of the incidence surface 103 of the light guide plate 102 are easy to leave portions of the emission surface 106 in the vicinity of the incidence surface as brightly shining lines (bright lines) 111 (are easy to be visually recognized). In recent years, such problems of the surface light source device 101 are highlighted in the industry since it is required to increase the effective emission area of the light guide plate 102 (the area of the emission surface capable of being used for emitting uniform surface illuminating light) as large as possible in order to increase the size of the display screen.